Various devices for detecting that a person has fallen off of a ship and for locating that person have been previously developed. All of these devices have significant shortfalls. Conventional inventions in this field can be classified by (1) the method used to transmit a signal from a sensor to a central receiver, or (2) the method in which the ship's personnel is alerted of the loss overboard. Most of these devices rely on radio frequency (RF) signals which propagate through the air, or on acoustic or ultrasonic signals propagating through the water. Water propagation of signals is not practical because it requires costly detection equipment not often found on a ship. RF transmission systems generally fail to produce a signal of sufficient strength to penetrate the air/water interface and still effectively alert those on the ship of the person overboard. Further, no conventional system interfaces with ship navigational equipment for recording event location.
The following four areas are of particular relevance to this type of detection system: mode of signal propagation, type of signal transmitted, complexity of the system, and the ability to determine the location of the individual who has fallen overboard.
Mode of propagation--This type of system is designed to detect the event of an individual who has fallen from a ship and into the water, therefore it must include a number of sensors, one sensor attached to each individual to be monitored, and the ability to signal the ship from the water. To signal the ship, a transmitter associated with the overboard individual can use radio waves, light, sound, or ultrasonic energy. In conventional systems, all forms of acoustic/ultrasonic energy must travel through the water to reach a sensor/receiver on the ship. Transmission through this medium is very unreliable, and the dependability of such a signal is highly dependent on the environment and on ambient noise. Reverberation effects from the water's surface, bottom, and thermal layers often mask this type of signal, or cause false alarms. Because this device is intended to provide an added measure of safety for those aboard the ship, high assurance is required. False alarms will cause the crew to react more slowly in the event of a real emergency.
Several systems are available which use lights, such as strobes or flares, to signal a ship in the event of a man overboard. These systems are inexpensive to put in place and are generally reliable. However, they require that a crew member must always be watching to observe the signal, because there is no practical way to automate this type of alarm.
Other systems, which signal the ship using a radio transmitter, fail to address the problem of the air/water interface and its effect on propagation. In low power RF devices such as those typically used in this type of system, the design and location of the antenna is one of the most important considerations in determining the range and quality of signal transmitted. In designing an event detector for this type of system, a transmitter and a water detector both must be located on or near the individual. If the transmitter is submerged for activation, and the antenna is below the air/water interface, the signal will not propagate the distance required to the ship with signal strength sufficient to deliver meaningful information.
Type of signal--Alarms on board the ship can be triggered by a signal, or by the absence of a signal indicating that everything is normal. Systems which depend on the interruption of a signal are generally not acceptable for ships with large numbers of people on board, since each individual would need a signal source and each signal source would have to be monitored. Additionally, this type of device would have to be active at all times, causing a drain on power supplies. This type of signal device would also cause many false alarms, since interference and environment can occasionally cause signals to drop out.
Complexity--Systems which use sonar or ultrasonic energy to transmit signals require significant amounts of equipment. Sonar sensors need to be mounted away from the ship's hull and other sources of noise, interference, and multipath. Additionally, acoustic and ultrasonic signals require complicated signal processing to determine the nature of the signal and to separate the signal from ambient noise.
Localization--Conventional systems signal the event of a person falling overboard but do not record the location and exact time of the event, and therefore cannot accurately plot the person's position. Systems which send a signal for localization are limited because the ship must be in range of the transmitter at all times or risk permanently losing the individual.